Lactic acid
Lactic acid is an organic acid with the molecular formula C3H6O3. In its solid state, it is white and miscible with water. When dissolved, it forms a colorless solution. Production includes both artificial synthesis and natural sources. Lactic acid is an alpha-hydroxy acid due to the presence of a hydroxyl group adjacent to the carboxyl group. It is a synthetic intermediate in many organic synthesis industries and in various biochemical industries. The conjugate base of lactic acid is called lactate. The name of the derived acyl group is lactoyl.
In solution, it can ionize by a loss of a proton to produce the lactate ion, also known as 2-hydroxypropanoate. Compared to acetic acid, its pK is 1 unit less, meaning that lactic acid is ten times more acidic than acetic acid. This higher acidity is the consequence of intramolecular hydrogen bonding between the α-hydroxyl and the carboxylate group.
Lactic acid is chiral, consisting of two enantiomers. One is known as -lactic acid, -lactic acid, or -lactic acid, and the other, its mirror image, is -lactic acid, -lactic acid, or -lactic acid. A mixture of the two in equal amounts is called -lactic acid, or racemic lactic acid. Lactic acid is hygroscopic. -Lactic acid is miscible with water and with ethanol above its melting point, which is. -Lactic acid and -lactic acid have a higher melting point. Lactic acid produced by fermentation of milk is often racemic, although certain species of bacteria produce solely -lactic acid. On the other hand, lactic acid produced by fermentation in animal muscles has the enantiomer and is sometimes called "sarcolactic" acid, from the Greek, meaning "flesh".
In animals, -lactate is constantly produced from pyruvate via the enzyme lactate dehydrogenase in a process of fermentation during normal metabolism and exercise. It does not increase in concentration until the rate of lactate production exceeds the rate of lactate removal, which is governed by a number of factors, including monocarboxylate transporters, concentration and isoform of LDH, and oxidative capacity of tissues. This reaction is reversible and redox-linked: LDH reduces pyruvate to lactate using NADH as an electron donor, simultaneously regenerating NAD⁺ required for glycolysis under anaerobic conditions. The concentration of blood lactate is usually at rest, but can rise to over 20mM during intense exertion and as high as 25mM afterward. In addition to other biological roles, -lactic acid is the primary endogenous agonist of hydroxycarboxylic acid receptor 1, which is a G protein-coupled receptor.
In industry, lactic acid fermentation is performed by lactic acid bacteria, which convert simple carbohydrates such as glucose, sucrose, or galactose to lactic acid. These bacteria can also grow in the mouth; the acid they produce is responsible for the tooth decay known as cavities. In medicine, lactate is one of the main components of lactated Ringer's solution and Hartmann's solution. These intravenous fluids consist of sodium and potassium cations along with lactate and chloride anions in solution with distilled water, generally in concentrations isotonic with human blood. It is most commonly used for fluid resuscitation after blood loss due to trauma, surgery, or burns.
Lactic acid is produced in human tissues when the demand for oxygen is limited by the supply. This occurs during tissue ischemia when the flow of blood is limited as in sepsis or hemorrhagic shock. It may also occur when demand for oxygen is high, such as with intense exercise. The process of lactic acidosis produces lactic acid, which results in an oxygen debt, which can be resolved or repaid when tissue oxygenation improves.
History
Swedish chemist Carl Wilhelm Scheele was the first person to isolate lactic acid in 1780 from sour milk. The name reflects the lact- combining form derived from the Latin word wikt:lac#Latin, meaning "milk". In 1808, Jöns Jacob Berzelius discovered that lactic acid is also produced in muscles during exertion. Its structure was established by Johannes Wislicenus in 1873.In 1856, the role of Lactobacillus in the synthesis of lactic acid was discovered by Louis Pasteur. This pathway was used commercially by the German pharmacy Boehringer Ingelheim in 1895.
Due to a combination of geographic and infrastructural factors, the Soviet Union, as well as several other members of the Warsaw Pact, experienced chronic shortages of citric and malic acid, among others. In order to combat this issue, the Narkomzem invested heavily in the development of suitable lactobacillus strains, which were able to produce lactic acid with relatively high efficiency from crude molasses feedstock. Despite synthetic citric acid being produced in some quantities across the Warsaw Pact, it proved far more difficult to purify, leading to lactic acid being, on average, a quarter of the cost of citric acid. The continued use of lactic acid in some Eastern European and Central Asian food production in the modern day, in favor of the more common citric or malic acids, lends it a distinctive flavor.
Global demand for lactic acid continues to expand, with an estimated annual growth rate of 5–8% driven by the increasing use of biodegradable plastics, green solvents, and pharmaceutical intermediates. Worldwide production exceeded 1.5 million tonnes by the early 2020s, up from roughly 275,000 tonnes in 2006, and is projected to keep rising as biobased materials replace petroleum-derived products. Major producers include NatureWorks LLC, Purac, Galactic, and several Chinese manufacturers. NatureWorks operates one of the world’s largest polylactic acid facilities in Blair, Nebraska, with a production capacity of about 140,000 tonnes per year, supplying feedstock for a wide range of biodegradable packaging and fiber applications.
Production
Lactic acid is produced industrially by bacterial fermentation of carbohydrates, or by chemical synthesis from acetaldehyde., lactic acid was produced predominantly by fermentation. Production of racemic lactic acid consisting of a 1:1 mixture of and stereoisomers, or of mixtures with up to 99.9% -lactic acid, is possible by microbial fermentation. Industrial production of the D-lactic acid enantiomer is technically more challenging because most naturally occurring lactic acid bacteria preferentially produce the L-form; obtaining high optical purity of D-lactic acid therefore requires genetically engineered microorganisms or specific D-lactate dehydrogenases.Fermentative production
are obtained industrially by fermentation of milk or whey by Lactobacillus bacteria: Lactobacillus acidophilus, Lacticaseibacillus casei, Lactobacillus delbrueckii subsp. bulgaricus, Lactobacillus helveticus, Lactococcus lactis,'' Bacillus amyloliquefaciens, and Streptococcus salivarius subsp. thermophilus.As a starting material for industrial production of lactic acid, almost any carbohydrate source containing and can be used. Pure sucrose, glucose from starch, raw sugar, and beet juice are frequently used. Lactic acid producing bacteria can be divided in two classes: homofermentative bacteria like Lactobacillus casei and Lactococcus lactis'', producing two moles of lactate from one mole of glucose, and heterofermentative species, producing one mole of lactate from one mole of glucose, as well as carbon dioxide and acetic acid/ethanol.
Chemical production
Racemic lactic acid is synthesized industrially by reacting acetaldehyde with hydrogen cyanide and hydrolysing the resultant lactonitrile. When hydrolysis is performed by hydrochloric acid, ammonium chloride forms as a by-product; the Japanese company Musashino is one of the last big manufacturers of lactic acid by this route. Synthesis of both racemic and enantiopure lactic acids is also possible from other starting materials by application of catalytic procedures.Biology
Molecular biology
-Lactic acid is the primary endogenous agonist of hydroxycarboxylic acid receptor 1, a G protein-coupled receptor.Metabolism and exercise
During power exercises such as sprinting, when the rate of demand for energy is high, glucose is broken down and oxidized to pyruvate, and lactate is then produced from the pyruvate faster than the body can process it, causing lactate concentrations to rise. The production of lactate is beneficial for NAD+ regeneration, which is used up in oxidation of glyceraldehyde 3-phosphate during production of pyruvate from glucose, and this ensures that energy production is maintained and exercise can continue. During intense exercise, the respiratory chain cannot keep up with the amount of hydrogen ions that join to form NADH, and cannot regenerate NAD+ quickly enough, so pyruvate is converted to lactate to allow energy production by glycolysis to continue.The resulting lactate can be used in two ways:
- Oxidation back to pyruvate by well-oxygenated muscle cells, heart cells, and brain cells
- * Pyruvate is then directly used to fuel the Krebs cycle
- Conversion to glucose via gluconeogenesis in the liver and release back into circulation by means of the Cori cycle
- * If blood glucose concentrations are high, the glucose can be used to build up the liver's glycogen stores.
The first stage in metabolizing glucose is glycolysis, the conversion of glucose to pyruvate− and H+:
When sufficient oxygen is present for aerobic respiration, the pyruvate is oxidized to and water by the Krebs cycle, in which oxidative phosphorylation generates ATP for use in powering the cell.
When insufficient oxygen is present, or when there is insufficient capacity for pyruvate oxidation to keep up with rapid pyruvate production during intense exertion, the pyruvate is converted to lactate− by lactate dehydrogenase), a process that absorbs these protons:
The combined effect is:
The production of lactate from glucose, when viewed in isolation, releases two H+. The H+ are absorbed in the production of ATP, but H+ is subsequently released during hydrolysis of ATP:
Once the production and use of ATP is included, the overall reaction is
The resulting increase in acidity persists until the excess lactate and protons are converted back to pyruvate, and then to glucose for later use, or to and water for the production of ATP.